Reactive metals such as titanium, zirconium, hafnium, molybdenum, chromium, niobium, high-temperature nickel-based super alloys, and other metals exhibit an intensive affinity towards oxygen and nitrogen, particularly when heated. In fact, titanium shows such an extreme affinity to oxygen that it is often employed as an oxygen "getter." When heating such metals and metal alloys for forming purposes, it is therefore necessary to do so under an atmosphere free of oxygen and nitrogen.
The metallurgical art has for some time recognized the desirability of utilizing induction heating methods for the melting of reactive metals, such as titanium, as a replacement for known industrial-scale melting processes based on, for example, consumable electrode arc-melting techniques. In induction melting, an electric current is induced into the metal to be melted. Thus, by supplying an alternating current to a primary induction coil, a reverse alternating current is induced into any electrical conductor lying within the magnetic field of the coil, producing heating in the conductor.
Typical induction heating processes are carried out in an oxygen-containing environment such as air. The presence of oxygen results in the formation of scale on the heated metal parts. Scale is an abrasive, which significantly contributes to the wearing of the forming dies, reducing their useful life.
There have been prior efforts to introduce an inert gas into the enclosures of various induction-heating apparatuses to eliminate, or at least substantially reduce, the presence of oxygen. In induction-heating apparatuses, where the induction coils and molten metal are contained in separate housings, a cover has been placed over the space between the housings to provide an airtight enclosure. Multiple inlets have been provided in the cover to transport an inert gas from a source into the pathway contained within the cover. The inert gas then diffuses into the housing to provide a more acceptable gaseous environment for induction heating and subsequent forming.
Disadvantages of such induction systems include lack of control of the injection of the inert gas and the inability to provide a barrier against the infiltration of unwanted gases, such as air, due to drafting. More specifically, induction-heating devices never achieve complete protection against air leaks. For example, it is known that air enters the induction heating apparatus though the entryway where the cold metal parts enter the apparatus and the exit where the heated parts leave the apparatus. In addition, air leaks may be present where the cover is attached to the housing of the induction heating apparatus. The infiltration of such air into the heating areas produces scaling.
Thus, there is a need for an induction-heating system capable of heating reactive metals in an environment of inert gas. A system is needed that is capable of eliminating air leaks and drafts associated with the loading and unloading of the metal billets. A significant benefit could be derived from a system capable of controlling the atmosphere and heating rate of reactive metal billets in an heating system.